1. Field of the Invention
The present general inventive concept relates to an image forming apparatus and a method thereof, and more particularly, to an image forming apparatus capable of protecting a system of the image forming apparatus when a fuser provided in the apparatus overheats, and a method thereof.
2. Description of the Related Art
A conventional image forming apparatus includes a charger, a laser scanning unit as an exposure unit, a developer, a transfer unit, a fuser, etc., around a photosensitive drum. A surface of the photosensitive drum charged by the charger is exposed to a beam of a laser scanning unit to form a latent image on the surface of the photosensitive drum. The developer develops the latent image into a toner image. The transfer unit transfers the toner image to a record medium. The record medium is discharged outside after the fuser fuses the toner image onto the record medium.
The fuser heats a sheet of paper carrying a toner image transferred thereto and temporarily fuses the powdery toner image on the sheet to fix the image to the sheet. To accomplish this, the fuser includes a fuser roller to fuse toner to a sheet of paper and a pressure roller to push the sheet toward the fuser roller. A heat lamp is provided in the fuser roller at its internal center. The fuser is heated by heat radiated from the heat lamp.
A thermistor is provided at a portion of the fuser roller at which it contacts the outer surface of the fuser roller to measure surface temperature of the fuser roller, and a thermostat is also provided to turn off power to the heat lamp when the surface temperature of the fuser roller has exceeded a preset level. The thermistor measures and transmits the surface temperature of the fuser roller to a microcomputer of the image forming apparatus. According to the measured surface temperature of the fuser roller, the microcomputer controls power supplied to the heat lamp to keep the surface temperature of the fuser roller within a predetermined temperature range. When the temperature of the fuser roller is higher than a preset upper limit, an internal connection of the thermostat is opened to turn off power supplied to the heat lamp.
As illustrated in FIG. 1, the conventional image forming apparatus includes a thermistor R4, an analog-to-digital (A/D) port (THERM_IN) of the microcomputer, an output port (nFUSER_EN) of the microcomputer, transistors Q1 and Q2, and a comparator U1A. The thermistor R4 senses surface temperature of the fuser roller. The microcomputer A/D converts and reads a voltage detected by the thermistor R4 through the A/D port (THERM_IN). The microcomputer compares the read voltage (Vadc2) with a fuser table previously stored in ROM and outputs a control signal to perform on/off control of the heat lamp through the output port (nFUSER_EN). The transistor Q2 is turned on or off according to the control signal. The transistor Q1 detects opening of the thermistor R4. The comparator U1A turns off the heat lamp when the fuser roller overheats.
The following is a description of how the conventional image forming apparatus of FIG. 1 operates. The microcomputer 31 A/D converts and reads the voltage detected by the thermistor R4 through the A/D port (THERM_IN). The microcomputer compares the read voltage with the fuser table previously stored in the ROM and determines whether to turn on or off the heat lamp. The microcomputer then outputs a value corresponding to the determination through the output port (nFUSER_EN). The voltage detected by the thermistor R4 is applied to a positive (+) input of the comparator U1A, which is a non-inverting input of the comparator U1A. The comparator U1A compares the detected voltage applied to the positive input with a reference level applied to the negative input, which is an inverting input of the comparator U1A. If the voltage applied to the positive input is lower than or equal to the reference level, the comparator U1A outputs a low signal. If the voltage applied to the positive input is higher than the reference level, the comparator U1A outputs a high signal. Accordingly, a voltage level associated with a predetermined temperature range is implemented using a reference level circuit so that the comparator U1A outputs a high signal in a normal operating state controlled at a temperature within an allowable temperature range, and outputs a low signal in an abnormal operating state controlled at a temperature outside the allowable temperature range. Accordingly, when the output of the comparator U1A is a high signal, the heat lamp is turned on or off according to an output value of the output port (nFUSER_EN) of the microcomputer. However, when the output of the comparator U1A is a low signal, the heat lamp is turned off regardless of the output value of the output port (nFUSER_EN) of the microcomputer. When the thermistor R4 is opened, the transistor Q1 connected to the thermistor R4 is turned off to switch the output of the comparator U1A to a low signal to prevent the risk of keeping the heat lamp on.
The following are some problems associated with the conventional image forming apparatus of FIG. 1. When the A/D port (THERM_IN) of the microcomputer is brought into an uncontrollable state, and thus the A/D (THERM_IN) port applies a certain voltage, the Vadc2 voltage becomes higher than the Vadc1 voltage. This causes a higher Vadc1 voltage than its original level to be applied to the non-inverting (positive) input of the comparator U1A through a damping resistor R3. That is, although the Vadc1 and Vadc2 voltages must have the same level under a normal condition, the level of the Vadc2 voltage is higher than that of the Vadc1 voltage to cause an increase in the level of the Vadc1 voltage. Since the level of the Vadc1 voltage is increased to be higher than the reference voltage applied to the inverting (negative) input of the comparator U1A, a normal operating state is detected to make it impossible to perform on/off control of the heat lamp through the comparator U1A. For example, since the A/D port (THERM_IN) of the microcomputer is damaged by external factors, even when the temperature of the fuser roller is very high, the voltage level of the positive input of the comparator U1A is higher than the reference voltage of the negative input so that the hardware control using the comparator U1A is deactivated to keep the heat lamp on. When the transistor Q2 is kept on according to the signal of the output port (nFUSER_EN) of the microcomputer, AC power is constantly applied to the heat lamp to continue heating the fuser roller until the thermostat is disconnected. In addition, even when the comparator U1A outputs a low signal, heat may scorch the fuser roller or disconnect the thermostat since the response to the temperature increase of the fuser roller is slow, thereby requiring the fuser roller to be repaired.
Thus, when an abnormal operation has occurred due to damage to the A/D port (THERM_IN) of the microcomputer, it is necessary to turn off the heat lamp, regardless of whether the microcomputer performs normal control, thereby preventing scorch of the fuser or disconnection of the thermostat, which causes inconvenience to the user.